PROJECT SUMMARY/ABSTRACT Lymphedema is a major complication after radiation and/or surgery for breast cancer. Unfortunately, there are no medications to effectively treat it. Serious complications include pain, lymphangitis, cellulitis, ulcers, and the development of malignant lymphangiosarcomas. Doxorubicin (DOX) is a central chemotherapeutic agent for treating breast cancer, but it increases the risk of lymphedema by 4- to 5-fold. The mechanism by which DOX contributes to the development of lymphedema is unknown, but it is thought to relate to its cytotoxic action. However, we propose that clinical concentrations of DOX directly inhibit the rhythmic contractions of lymph vessels (LVs) that propel lymph fluid from tissues back to the heart to prevent lymphedema. These contractions are tightly controlled by the calcium concentration and redox state in LVs. High-resolution in vivo imaging also reveals that systemically administered DOX profoundly reduces lymph flow. The suppression of LV contractile function by DOX is largely prevented by both dantrolene (DANT), a clinically available blocker of ryanodine receptors (RYRs), and MitoTEMPO, a mitochondrial-specific reactive oxygen species (mitoROS) scavenger. These data, along with knowledge that DOX activates RYRs and induces mitochondrial dysfunction in striated muscle and recent discoveries of functional RYRs in LVs, have led us to hypothesize that: DOX acutely opens RYRs to increase cytosolic calcium and mitoROS in lymph muscle cells (LMCs), resulting in the loss of LV contractions and inducing lymphostasis and lymphatic injury. Accordingly, we will explore whether DANT, an FDA-approved RYR blocker, can prevent DOX-induced lymphatic dysfunction. Three aims will use an integration of techniques to explore this hypothesis and will rely on protein and functional analysis of isolated lymph muscle cells and whole LVs, use optical imaging to assess volumetric lymph flow in vivo in response to DOX and RYR blockade, and investigate the utility of DANT as a potential therapeutic in a rat breast tumor model. Aim 1 will quantify the gene and protein expression profiles of RYR subtypes and determine whether DOX activates RYRs to increase cytosolic calcium and mitoROS generation as a mechanism of inhibiting LV contractions. Aim 2 will evaluate if DANT represents a potential therapeutic option to prevent DOX- induced suppression of lymph flow and prevent DOX-induced lymphatic injury. Aim 3 will investigate the effects of DANT on the anticancer activity of DOX in a rat model of breast cancer. Thus, we plan to explore RYRs as new therapeutic targets for DOX-related lymphedema and evaluate whether RYR blockers can be repurposed as anti-lymphedema medications.